DE 10 2012 020 908 A1 of the applicant already discloses a method of the aforementioned type for operating a motor vehicle with all-wheel drive, which has a permanently driven primary or front axle and a secondary or rear axle that can be enabled and disabled, the drive of which is enabled and disabled by a control unit as a function of a current driving situation. One of the essential advantages of such an all-wheel drive that can be enabled and disabled exists in the fact that the fuel consumption of the motor vehicle can be minimized by disabling the all-wheel drive when it is not required in order to thereby particularly minimize the power losses or the drag torque of the axle drive of the secondary or rear axle, which can be disabled.
The drive train of the motor vehicle in accordance with DE 10 2012 020 908 A1 comprises a so-called all-wheel clutch in the form of a clutch that is engaged when the all-wheel drive is enabled and the torque provided by the internal combustion engine is distributed to both axles of the motor vehicle and which is disengaged when the all-wheel drive is disabled and the torque provided by the internal combustion engine is only supplied to the permanently driven primary or front axle. The secondary or rear axle, which can be enabled, comprises a second clutch in the form of a separating clutch that is engaged and disengaged together with the all-wheel clutch. There are multiple rotating components between the two clutches, including a connecting shaft leading from the all-wheel clutch to the secondary or rear axle, the components of the all-wheel clutch connected to the connecting shaft on the output side in a torsionally resistant manner, as well as the axle drive of the secondary or rear axle, in addition to the equalization or limited slip differential and the rotating components thereof. In the engaged state of the two clutches, all of these rotating components are driven by the internal combustion engine. If the clutches are disengaged during travel, this leads to the circumstance that the aforementioned components are uncoupled from the drive train and rotate freely. Because the components are decelerated by an applied drag torque, their rotational speed gradually decreases to zero. The drag torque comprises decelerating torque components, which result, among other things, from the oil sloshing of a crown wheel of the axle drive of the secondary or rear axle as well as the bearing friction of the connecting shaft in the pivot bearings thereof, as well as accelerating torque components, which result, among other things, from a rotational speed difference between the input side and the output side components of the oil-filled all-wheel clutch as well as from the friction in the limited slip differential.
Upon the next enabling of the all-wheel drive, the components previously uncoupled from the drive train must be re-accelerated and/or brought back to rotational speed, wherein the necessary acceleration and/or the torque of the internal combustion engine required for acceleration depend(s) on the current rotational speed of the uncoupled components and the applied drag torque.
For this reason, the knowledge of the drag torque applied when the all-wheel drive is disabled is important in order to be able to adapt the rotational speed of the uncoupled and freely rotating or stopped components when the disabled secondary or rear axle is enabled as quickly as possible, on one hand, to the rotational speed of the remaining components of the drive train without this resulting in a noticeable jolt in the drive train and thus in the motor vehicle, on the other hand. In order to achieve enabling with optimum comfort, the acceleration must occur more slowly in other words, the larger the drag torque or the drag losses are that are counteracting an acceleration.
The applied drag torque depends on multiple parameters, namely the rotational speed of the components, the oil level in the clutches and in the axle drive, the temperature of the lubricating oil in the axle drive, as well as on the run-in and/or wear of the bearings and seals. The oil level in the clutches and in the axle drive is known, for example, while the rotational speed and the lubricating oil temperature can be measured. The run-in state and/or wear of the bearings and seals are unknowns that must be estimated. The applied drag torque is determined at present for various driving states as a function of the aforementioned parameters on a test bench and permanently stored in the control unit of the motor vehicle.
In order to save costs however, sensors should increasingly be dispensed with, also including a sensor to measure the lubricating oil temperature in the axle drive of the secondary or rear axle, which can be disabled. However, if further unknowns occur in addition to the run-in and/or wear of the bearings and seals, such as, for example, the lubricating oil temperature or the oil level in the axle drive, a meaningful statement regarding the applied drag torque is hardly even possible.